Defective CD4 T cell responses are associated with disease progression in HIV-infected persons, and the mechanisms responsible for this dysfunction are poorly understood. A fundamental question is whether CD4 T cell impairment is due to an irreversible loss of functions or under control of inhibitory mechanisms that may be reverted by manipulation of regulatory pathways. Recent findings provide evidence that major defects in CD4 function in HIV+ subjects may be reversible: we and others showed that the Programmed Death 1 (PD-1) pathway plays a key role in T cell dysfunction in vitro, and that in persons with HIV infection, PD-1 expression on CD4 T cells is associated with disease progression. Our published report, for which the PI is the co-first author, and preliminary data in this application demonstrate that CD4 T cell proliferative capacity can be restored by blocking the interaction of PD-1 with its ligand PD-L1. In addition, our preliminary data show that expression of another inhibitory receptor, CTLA-4, is also a marker for reversible CD4 T cell dysfunction. In this application, we propose to elucidate the role of PD-1 in the impairment of HIV-specific CD4 T cell proliferation in HIV+ subjects. In Aim 1, building on previous results correlating PD-1 expression by CD4 T cells with CD4 count and viral load, we will first assess expression by CD4 T cells of PD-1 and CTLA-4 in HIV+ subjects at various stages of disease. To determine the effect of varying levels of inhibitory molecule expression on T cell function, we will use quantitative RT-PCR and sorted HIV-specific CD4 T cell populations to define the phenotypic and functional profile of PD-1low and PD-1high virus-specific CD4 T cells. Given that involvement of gut-associated lymphoid tissue (GALT) is central to HIV pathogenesis, we will also assess the expression of PD-1 and its ligands in biopsies of gut-associated lymphoid tissue, and identify these molecules on various cell populations and on HIV-infected cells. In Aim 2, we will determine whether there is a correlation between the degree to which CD4 T cell dysfunction can be reversed and two parameters: disease status, and level of expression of inhibitory receptors. Preliminary data are consistent with a model in which some PD-L1+ subsets of APCs take up HIV antigen and present it to HIV-specific T cells in a manner that inhibits efficient proliferation. In Aim 3, using selective depletion, we will identify APC subsets in peripheral blood of HIV+ subjects that inhibit CD4 T cell proliferation via PD-1, and determine whether PD1 ligand can alter HIV-specific CD4 T cell function when present on non-antigen-loaded cells (in trans) or only if presented on the same cell as antigenic peptide (in cis). Our previous studies have shown that cells of the immune system become "exhausted" in people infected with HIV. We are studying the mechanisms responsible for this exhaustion and looking for ways to restore normal immune function in HIV-infected persons.